Tannin extraction

ABSTRACT

PCT No. PCT/AU90/00335 Sec. 371 Date Apr. 2, 1991 Sec. 102(e) Date Apr. 2, 1991 PCT Filed Aug. 7, 1990 PCT Pub. No. WO91/01989 PCT Pub. Date Feb. 21, 1991.Method for extracting tannins from bark and multi-stage apparatus suitable therefor. The method includes the steps of: contracting bark containing tannins with a first tannin enriched aqueous wash medium at elevated temperatures; separating the tannin-rich liquid extract by filtration including dewatering the resulting filter cake under pressure; washing the filter cake with a plurality of aqueous wash media at elevated temperatures; and separating the tannin enriched media after each washing stage by filtration including dewatering the filter cake under pressure; wherein each successive wash medium has a lower degree of tannin enrichment than the preceding wash medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the extraction of tannins from naturalsources and in particular to the extraction of tannins present in barkand wood extracts.

Tannins may be found in the bark and wood extracts of various varietiesincluding wattles (acacias) and pines (pinus). Such naturally occurringtannins are polyhydroxyphenols commonly referred to as polyphenols. Inpinus radiata bark for example the molecular weight of tannins can rangefrom less than 10³ to more than 10⁶.

Wattle tannins are generally of lower molecular weight and lowerviscosity which is generally believed to be due to the action of sugarsnaturally present in wattle tannins acting as viscosity reducers.

Pine tannins are generally more reactive than wattle tannins and areespecially suitable for use as wood adhesives, particularly in themanufacture of plywood and particle board. Other potential applicationsfor tannins include finger jointing wood adhesives, paper impregnation,leather tanning, dispersing clays, minerals and pigments, foundry corebinding and polyurethane production.

The higher molecular weight and higher viscosity portions of tannins areuseful in production of adhesives but are difficult to extract withconventional methods as they do not diffuse as readily as tannins oflower molecular weight. 2. Discussion of the Prior Art

Australian Patent Specification 533,791 discloses a method for producinga low viscosity material suitable for use in a formaldehyde-condensationadhesive which comprises subjecting a conventional aqueous extract ofbark or wood to ultrafiltration and separating out that fraction whichdoes not contain the high viscosity producing materials. There is nodetailed disclosure of the method or apparatus of extraction only areference to simple batch extraction of comminuted wood and bark withhot water at about 100° C. This conventional technique has thedisadvantage however of low extraction rates.

Australian Patent Specification 569,439 also refers to the low yields ofthe known aqueous extraction process and to the difficulties offormulating adhesives from 100° C. aqueous extracts. This specificationprovides also a method for producing a relatively low viscosity tanninextract involving separation and treatment of the high molecular weightmaterial with one or more sulphite compounds to reduce the molecularweight prior to recombining with the low molecular weight material.

Neither of these methods teach a method or apparatus for increasing theextraction yield.

Australian Patent Specification 579,783 also recognises the difficultieswhich have been encountered with conventional extraction techniques. Thedifficulties which have been encountered are attributed primarily toexcessive viscosity of the extracts and the difficulty in obtaininguniformity in product quality.

Specification 579,783 teaches a method for recovering tannin extractwhich comprises subjecting bark and/or wood to a first stage hot aqueoustannin extraction process, to obtain a first stage tannin extract. Thisis followed by a second stage hot aqueous extraction process at a higherpH than the first stage then recombining the first and second stageextracts. The specification further teaches the separation of highmolecular weight material and treatment thereof with one or moresulphite compounds to reduce the molecular weight.

The use of a two stage process and the introduction of sulphites resultin a more complex extraction method and increases the risk ofenvironmental contamination.

In Australian Patent Specification 518,703 the point is made thatalthough it is a relatively simple matter to prepare aqueous phenolicextracts from bark in the laboratory the phenolic materials contained inthe extracts are very reactive and considerable difficulty isexperienced in preventing or minimising premature reactions which makethe extracts obtained less useful. This specification refers to a hotwater extraction process as not favoured for use with most bark speciesprincipally on account of the uneconomically low yields of extractivesobtained.

Specification 518,703 teaches a method of treating an aqueous phenolicbark extract which involves adjusting the pH above 7 maintaining theextract temperature above 49° C. for a period of up to 90 minutes andadjusting the pH of the extract to below pH 7. This method requiresclose monitoring of the extract and addition of chemicals to adjust pH.Moreover, the treatment method proposed is independent of the particularmethod used to derive the extract from the bark and accordingly does notprovide increased extraction yields.

Accordingly a need exists for a novel method and apparatus for the moreefficient extraction of tannins from bark.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide, in one embodiment,a novel method for extracting tannins from bark.

It is a further object of the present invention to provide, in anotherembodiment, a novel apparatus for extracting tannins from bark.

The present invention provides in one embodiment a method for extractingtannins from bark including the steps of:

contacting bark containing tannins with a first tannin enriched aqueouswash medium at elevated temperatures;

separating the tannin-rich liquid extract by filtration includingdewatering the resulting filter cake under pressure;

washing the filter cake with a plurality of aqueous wash media atelevated temperatures; and

separating the tannin enriched media after each washing stage byfiltration including dewatering the filter cake under pressure;

wherein each successive wash medium has a lower tannin content than thepreceding wash medium.

According to a further embodiment the invention also provides amulti-stage apparatus for extracting tannin from bark comprising anextraction tank for slurrying fresh bark with a tannin enriched washmedium to effect primary extraction;

a filter for filtering the slurry after primary extraction and fordewatering the resulting filter cake under pressure;

at least one wash liquor holding tank for providing a tannin enrichedliquor for washing a filter cake in the filter;

a source of hot water for providing a final medium for washing thefilter cake in the filter; and

pumping means for moving the slurry and/or liquid media between thetanks and the filter

wherein the filter is adapted for dewatering the filter cake underpressure between each separation and/or wash step.

The invention may also include a tannin rich recycle circuit. In thisarrangement the tannin rich stream flowing from the filter may beseparated whereby a portion thereof is recycled preferably via a holdingtank or header tank to facilitate forming a pumpable slurry with freshbark and whereby the remainder thereof is recovered from the system as atannin rich extract. The removed portion may optionally undergoconcentration whereby to form a concentrated extract.

The bark provided as a raw material for use according to the inventionis preferably comminuted before introduction into the extraction tank.The introduced bark may be milled bark and is preferably finely milledso as to facilitate solid liquid contact and thereby improve theextraction rate. As higher molecular weight tannins do not readilydiffuse it is preferable if only a smaller percentage, preferably notmore than 5% of the bark is greater than 1 mm in size. In a typical runof mill bark sample at least 20% of the sample may be less than 125microns.

A typical distribution for run of mill pinus radiata bark suitable foruse in accordance with the present invention is as follows:

    ______________________________________                                        Less than 125 microns                                                                            43%                                                        More than 125 microns                                                                            19.5%                                                      More than 212 microns                                                                            16.9%                                                      More than 355 microns                                                                            13.6%                                                      More than 600 microns                                                                            3.9%                                                       More than 1 mm     0.5%                                                       ______________________________________                                    

Although the present invention is particularly suitable for applicationto pinus radiata bark it is to be appreciated that the invention mayalso be applied to other barks and to other tannin containing woodproducts.

The filter provided in accordance with the invention is preferably apressure filter of the plate and frame type. Preferably the filterincorporates the means for dewatering the filter cake under pressure. Itis to be appreciated that the filtration and the dewatering may occursimultaneously. In some cases a degree of filtering may occur as theslurry is pumped into the filter and further dewatering may occur afterthe increased pressure is applied.

It is important for efficient operation of the invention that thedewatering of the filter cake after separation of each liquid extract isas complete as possible. Although tannins dissolve fairly readily in hotwater it has been found that moist bark such as the bark residue afterfiltration can retain more than 100% or even 150% and typically around200% of moisture by weight of dry bark. If such moisture is notminimised a considerable amount of dissolved tannin will not beseparated into the liquid extracts. It is particularly preferred fordewatering to reduce the moisture content of the filter cake to lessthan 100% or if possible 50% by weight of dry bark.

It has been found particularly preferable if the filter applies apressure of at least 10 bar and preferably a pressure in the range of 12to 16 bar to the filter cake during dewatering.

A fully automatic pressure filter is preferred for use in accordancewith the present invention and it has been found that a filter with adiaphragm pressing system is particularly suitable for use in thepresent invention. One such filter which we have found particularlysuitable is the Larox pressure filter.

The primary extraction of tannins from comminuted bark may be conductedin an extraction tank. The bark and wash liquor may be slurried in theextraction tank at elevated temperatures. Preferably the tannin enrichedwash medium is at a temperature of at least 60° C. and most preferablybetween 90° and 100° C. when brought into contact with the bark.

The present invention operates on a counter current multistage washingsequence wherein each successive wash medium has a lower tannin contentthan the preceding wash medium. The final wash medium is preferablywater having no tannin enrichment.

It has been found convenient if each wash medium is stored in a holdingtank at elevated temperatures preferably above 60° C. and mostpreferably between 90° and 100° C. between each wash cycle. Accordinglyit is preferred if each holding tank has heating means and/or insulationassociated therewith.

It has been found that the method provided by the present inventiongives improved results if the milled bark and enriched wash liquor areslurried in the extraction tank at elevated temperatures for an ageingperiod prior to initial separation in the filter. An ageing period ofbetween 10 minutes and 1 hour, preferably between 15 minutes and 45minutes and most preferably between 20 minutes and 40 minutes has beenfound effective in increasing the rate of extraction of tannins inaccordance with the invention.

To avoid the filter lying idle during the ageing period a secondextraction tank may be provided in accordance with the invention wherebythe slurried bark and wash liquor in the second tank may undergo ageingwhile the contents of the first tank is undergoing separation andwashing and vice versa.

The apparatus provided in accordance with the present invention may alsoinclude agitating means for retaining the slurry in suspension in theextraction tank. Preferably the apparatus also includes insulation meansfor retaining liquids in the system at elevated temperatures.

As tannins tend to chelate with the iron in mild steel causing darkeningof the colour of the extract and blackening of the equipment it ispreferred if the apparatus of the invention is constructed fromstainless steel, plastics or other inert materials.

The source of hot water may be a holding tank adapted to be refilledwith water as required and preferably adapted for heating water to thedesired elevated temperature.

Preferably the bark undergoes a plurality of washing stages followinginitial separation of the slurry. There may be at least 2 preferably 3and most preferably between 2 and 4 and not more than 5 washing stagesfollowing initial separation of the slurry. The wash medium used in eachstage has a lower tannin content than that of the preceding stage. Thefinal wash medium may be unenriched water.

For use in particle board manufacture a solids content of at least 30%tannin and preferably from 35 to 45% is preferred in the final extract.The tannin rich extract from the filter may accordingly be passedthrough an evaporator to form a concentrated extract of the desiredconcentration. Conventional evaporators such as falling film, forcedcircular or plate evaporators are suitable for use in accordance withthe invention.

The extraction method and apparatus provided according to the inventionhas been found to be efficient and adaptable in that by varying theamount of wash liquor employed the tannin concentration in the finalextract may vary. For example if the volume of wash liquor is increasedthe volume of extract will increase but the concentration will reduce.

The spent bark after undergoing tannin extraction in accordance with theinvention may be used as fuel or for horticultural or otherapplications.

A preferred embodiment of the invention will now be described inrelation to the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 comprises a schematic representation of a tannin extractionapparatus in accordance with the present invention.

DETAILED DESCRIPTION

Fresh wash hot water is introduced into the apparatus via holdingtank 1. Faucet means is provided for topping up tank 1 with fresh water.Tank 1 is also provided with heating means to facilitate water beingdrawn from tank 1 at elevated temperatures preferably between 95° and100° C.

Extraction of tannin according to the present invention is achieved bysuccessive washing with progressively more dilute solutions at elevatedtemperatures with filtration and dewatering between each washing stageuntil the strength of the enriched solutions decreases below apredetermined level. The hot water from tank 1 comprises the final washsolution.

Holding tanks 2 to 5 progressively hold tannin enriched liquors ofincreasing concentration. Each tank may be insulated and/or may containheating means to facilitate wash media being drawn from the tank at anelevated temperature preferably in the range of from 95° to 100° C.

Tank 6 comprises an extraction tank for slurrying fresh comminuted barkwith the wash liquor from tank 5 which is a tannin enriched wash medium.

In the operation of the apparatus shown in FIG. 1 according to themethod provided by the present invention finely comminuted bark amajority of which is preferably -1 mm in size, is added to extractiontank 6. An extraction solution being tannin enriched wash liquors fromtanks 4 and 5 is added to tank 6. After the bark and extraction solutionis aged in tank 6 under agitation at elevated temperature for a periodof not more than 1 hour and preferably for a period of from 20 to 40minutes, the slurry is pumped to the pressure filter in which the finalrich liquor is separated from the bark. The final rich liquor which maycontain approximately 10 to 15% tannin depending upon the amount of washwater introduced per cycle is divided into two streams. One stream ispassed via paths 9 and 6 to rich recycle liquor tank 5. This streamforms a rich liquor recycle circuit within the apparatus of theinvention. The other stream which may comprise the balance of the richliquor extract from the pressure filter passes through rich extractholding tank 7 via paths 10 and 11 to be concentrated in the evaporator.The concentrated extract from the evaporator may have a tannin solidscontent of at least 30% and preferably around 40%. The extract is passedto holding tank 8 via path 12. The removed moisture may pass through acondensor via path 13 before discharge from the system.

The filter cake remaining in the pressure filter is washed by a washliquor from tank 4 which in a typical example may increase fromapproximately 2-3% tannin as it proceeds via path 4 to the filter toapproximately 12% tannin as it proceeds via path 6 to holding tank 5 andsubsequently extraction tank 6.

Holding tank 5 acts as a header tank for enriched wash liquor and isused for withholding the wash liquor while wash liquor from a precedingcycle is ageing with bark in tank 6 and being fed into the filter.Holding tank 5 accordingly forms part of a rich liquor recycle circuit.

It is to be appreciated that the rich recycle liquor in tank 5 and richextract in tank 7 have the same tannin content as tanks 5 and 7 arefilled by a separation of the tannin rich stream flowing from the filtervia path 9. The internal rich liquor recycle so formed increases theamount of liquid in tank 6 preferably to a level of 5 parts liquid to 1part bark which facilitates forming a pumpable slurry with the bark.

Tank 5 accordingly acts as a holding tank for that proportion of thefilter extract to be recycled. It is preferred to use a holding tankrather than to introduce the recycle stream directly into tank 6 as theextract to be recycled in any particular cycle may commence flowing fromthe filter along paths 9 and 6 before tank 6 is emptied of slurry.

After separation of the liquid phase from tank 6 in the pressure filterto provide the filter cake, the cake may be washed successively by thewash liquors in tanks 4, 3, 2 and 1 via paths 4, 3, 2 and 1respectively.

In a typical arrangement the wash liquors held in tanks 4, 3 and 2 mayrespectively hold approximately 2-3, 1 and 0.5% tannin.

Although not shown in FIG. 1 a second extraction tank may be provided toenable ageing of a bark and concentrated wash liquor slurry in one tankwhile material from the other extraction tank is undergoing multi-stagewashing. The second extraction tank may be filled with enriched washmedia from the pressure filter via an alternative path 7 and from washliquor tank 5 via an alternative path 6.

Fresh bark is added to the system via path 20 to extraction tank 6.

By finely milling or otherwise comminuting the bark prior to addition toextraction tank 6 via path 20 the tannin extractions facilitated and apumpable slurry is formed.

It may be necessary to pre-dry the bark to restrain tanninpolymerisation through a composting action which may occur if the barkis left in large moist stockpiles. Similarly, if the bark is notpre-dried prior to addition to the system excessive moisture which maybe up to 120% or more by weight would effectively dilute the richextract proceeding via path 10 to tank 7 and thereby reduce theeffectiveness of the invention.

If drying is required prior to addition of fresh bark via path 20 asimple hot air dryer would be suitable. Such a dryer may be fired atleast partly by spent bark removed from the system via path 8.

Preferably each of tanks 1 to 6 is insulated and/or heated and all pipesare preferably lagged whereby to maintain all liquids upstream of richextract tank 7 at elevated temperatures.

Older butt log bark has a higher tannin content than younger strand barkhowever the younger bark generally contains tannins of a lower molecularweight and is therefore easier to extract.

The nature of the bark accordingly determines the molecular weight ofthe tannins produced in accordance with the invention. One of theadvantages provided by the present invention is that the extraction ofnaturally occurring tannins from bark may be carried out without theaddition of any reagents other than water. However, additives may beintroduced into the system for example if the molecular weightcharacteristics of the final product are to be altered. Additives may beadded to extraction tank 6 when fresh bark is introduced or may be addedelsewhere into the system.

A small amount of phenol, preferably not more than 5% and mostpreferably around 3% on tannin solids may be added to thepre-concentrate to reduce concentrate viscosity. Such an additiveaddition may retard mold growth in the concentrated extract which mayotherwise be encouraged by the presence of sugars, gums, etc. in theextract.

The spent bark after the final washing and dewatering stage may beremoved from the filter via path 8 and may be used for nursery andhorticultural applications. The spent bark may also be a suitable fuelfor example to provide hot gases for drying fresh bark therebyconserving energy within the system.

The method and apparatus provided by the present invention provides amechanical extraction method which avoids degrading the tannin moleculesby chemical attack and also provides for high tannin extractionefficiencies without using large quantities of liquid. The low water/drybark ratio preferably in the vicinity of 3:1 results in a highconcentration extract preferably in the region of 10 to 15%. The use ofa water/dry bark ratio in the region of 3.5:1 would generally not besufficient to form a pumpable slurry suitable for extraction however byrecycling a portion of the rich extract this can be achieved inaccordance with the present invention. The present invention accordinglyfacilitates the production of a high concentration tannin extractwithout the need for extensive evaporation. Further, by using only hotwater for the extraction no potentially injurious chemical salts areintroduced.

Bark can be rather bulky and difficult to transport. The presentinvention has the added advantage that movement of bark is kept to aminimum.

While it has been convenient to describe the invention herein inrelation to particularly preferred embodiments, it is to be appreciatedthat other constructions and arrangements are also considered as fallingwithin the scope of the invention. Various modifications, alterations,variations and/or additions to the constructions and arrangementsdescribed herein are also considered as falling within the scope andambit of the present invention.

I claim:
 1. A method for extracting tannins from tannin-containing bark;said method comprising the steps of:(a) contacting tannin-containingbark with an amount of a first tannin-enriched aqueous wash medium for alength of time sufficient to extract the tannins to form a slurry of atannin-rich liquid extract and extracted bark; said tannin-enrichedaqueous wash medium having a temperature within the range of 60°-100°C.; (b) separating the tannin-rich liquid extract from the extractedbark by pressure filtration; said step of separating comprising:(i)filtering the tannin-rich liquid extract from the extracted bark to forma filter cake of extracted bark and retained tannin-rich liquid; and(ii) dewatering the resulting filter cake under a sufficient pressure todecrease the amount of retained tannin-rich liquid in the filter cake;(c) subjecting the filter cake to two subsequent washing and separatingstages, wherein:(i) each washing stage comprises washing the filter cakewith an amount of a tannin-enriched aqueous wash medium having atemperature of 60°-100° C. to extract therein tannin remaining in thefilter cake and to form a tannin-rich liquid extract; saidtannin-enriched aqueous wash medium having a lower degree of tanninenrichment than each previous tannin-enriched aqueous wash medium; and(ii) each separating stage comprises separating the tannin-rich liquidextract from the filter cake after each washing step by pressurefiltration; said step of separating comprising: (A) filtering thetannin-rich liquid extract from the filter cake; and (B) dewatering thefilter cake under a sufficient pressure to decrease the amount ofretained tannin-rich liquid in the filter cake; and (d) collecting thetannin-rich liquid extract resulting from each washing and separatingstages.
 2. The method according to claim 1, further comprising a step ofwashing the filter cake with water after the step of subjecting thefilter cake to two subsequent washing and separating stages to extracttherein tannin remaining in the filter cake; said water having atemperature within the range of 60° C. to about 100° C. and containingno tannins.
 3. The method according to claim 1, wherein the firsttannin-enriched aqueous wash medium contains a portion of thetannin-rich liquid extract.
 4. The method according to claim 1, whereinthe tannin-rich liquid extract is subjected to concentration byevaporation to form a concentrated extract,
 5. The method according toclaim 1, where the tannin-containing bark comprises Pinus Radiata bark.6. The method according to claim 1, wherein the tannin-containing barkis comminuted before contact with the first tannin-enriched aqueous washmedium.
 7. The method according to claim 6, wherein no more than 5% ofthe tannin-containing bark is greater than about 1 mm in any dimension.8. The method according to claim 1, wherein each step of dewatering thefilter cake is conducted under a pressure of at least about 10 bar toabout 16 bar.
 9. The method according to claim 1 wherein thetannin-containing bark and the first tannin-enriched aqueous wash mediumare slurried at a temperature within the range of about 60° to 100° C.for a period of 10 minutes to 1 hour.
 10. The method according to claim1, wherein the filter cake is subjected to two to four subsequentwashing and separating stages.
 11. The method according to claim 10,wherein each washing and separating stage occurs within a filter withoutdisplacement of the filter cake from the filter.
 12. A method forextracting tannins from tannin-containing bark; said method comprisingthe steps of:(a) contacting tannin-containing bark with an amount of afirst tannin-enriched aqueous wash medium at a temperature of 60°-100°C. for a length of time sufficient to extract the tannins to form aslurry of a first tannin-rich liquid extract and extracted bark; (b)separating the first tannin-rich liquid extract from the extracted bark;said step of separating comprising:(i) filtering the first tannin-richliquid extract from the extracted bark with a pressure filter to form afilter cake of extracted bark and retained tannin-rich liquid; and (ii)dewatering the resulting filter cake under a pressure within the rangeof about 10 bar to a maximum pressure that the presure filter canwithstand to decrease the amount of retained tannin-rich liquid in thefilter cake; (c) washing the filter cake with an amount of a secondtannin-enriched aqueous wash medium at a temperature of 60°-100° C. toextract the tannins to form a second tannin-rich liquid extract; saidsecond tannin-enriched aqueous wash medium having a lower degree oftannin enrichment than said first tannin-enriched aqueous wash medium;(d) separating the second tannin-rich liquid extract from the filtercake; said step of separating comprising:(i) filtering the secondtannin-rich liquid extract from the filter cake with the pressurefilter; and (ii) dewatering the filter cake under a pressure within therange of about 10 bar to a maximum pressure that the pressure filter canwithstand to decrease the amount of retained tannin-rich liquid in thefilter cake; (e) washing the filter cake with an amount of a thirdtannin-enriched aqueous wash medium at a temperature of 60° to 100° C.to extract the tannins to form a third tannin-rich liquid extract; saidthird tannin-enriched aqueous wash medium having a lower degree oftannin enrichment than said second tannin-enriched aqueous wash medium;(f) separating the third tannin-rich liquid extract from the filtercake; said step of separating comprising:(i) filtering the thirdtannin-rich liquid extract from the filter cake with the pressurefilter; and (ii) dewatering the filter cake under a pressure within therange of about 10 bar to a maximum pressure that the pressure filter canwithstand to decrease the amount of retained tannin-rich liquid in thefilter cake; (g) washing the filter cake with an amount of watercontaining no tannin therein at a temperature within the range of 60° C.to 100° C. to extract the tannins to form a final liquid extract; and(h) separating the final liquid extract from the filter cake; said stepof separating comprising:(i) filtering the final liquid extract from thefilter cake with a pressure filter; and (ii) dewatering the filter cakeunder a pressure within the range of about 10 bar to a maximum pressurethat the pressure filter can withstand to decrease the amount ofretained liquid in the filter cake; and (i) collecting the firsttannin-rich liquid extract, second tannin-rich liquid extract, thirdtannin-rich extract, and final tannin-rich extract.
 13. The methodaccording to claim 12, wherein the pressure is within the range of about10 bar to about 16 bar.
 14. An apparatus for extracting tannins fromtannin-containing bark comprising:(a) a first extraction tank forslurrying tannin-containing bark with a tannin-enriched aqueous washmedium to form a slurry of a tannin-rich liquid extract and extractedbark, said tank having a means for adding the tannin-containing bark, ameans for adding the tannin-enriched aqueous wash medium, and a meansfor discharging the slurry; (b) a pressure filter connected to the meansfor discharging the slurry for fitering the discharge slurry to separatethe tannin-rich liquid extract from the extracted bark to form a filtercake and to dewater the filter cake, said pressure filter having a meansfor applying pressure on the filter cake; (c) one wash liquor holdingtank connected to the pressure filter for providing a tannin-enrichedaqueous wash medium for washing the filter cake on the filter to form atannin-rich liquid extract; (d) a source of water connected to thepressure filter for providing a final medium for washing the filter cakein the filter to form a tannin-rich liquid extract; (e) a tannin-richextract collection tank for holding the tannin-rich liquid extractsflowing from the pressure filter and connected to the pressure filter;and (f) pumping means coupled to the tanks and the pressure filter formoving the slurry and the liquid media between the tanks and thepressure filter.
 15. An apparatus according to claim 14, wherein theextraction tank includes agitating means for retaining the slurry insuspension.
 16. An apparatus according to claim 14 further comprising atannin-rich recycle circuit connected to the pressure filter and theextraction tank and adapted for separating a tannin-rich stream flowingfrom the pressure filter and for recycling a portion of the stream tothe extraction tank.
 17. An apparatus according to claim 14 in which themeans for applying pressure on the filter cake applies a pressure withinthe range of about 10 bar to a maximum pressure that the pressure filtercan withstand.
 18. An apparatus according to claim 17, wherein the meansfor applying pressure on the filter cake applies a pressure within arange of about 10 bar to about 16 bar.
 19. An apparatus according toclaim 14, further comprising a second extraction tank for slurrying andaging tannin-containing bark and a tannin-enriched aqueous wash mediumwhile the contents of the first extraction tank undergo separation andwashing, said second extraction tank having means for addingtanning-containing bark, a means adding a tannin-enriched aqueous washmedium, and a means for discharging the slurry connected to the pressurefilter.
 20. The apparatus according to claim 14, further comprisingthree wash liquor holding tanks connected to the pressure filter forproviding wash media of successively lower tannin contents.
 21. Anapparatus according to claim 14 further comprising an evaporatorconnected to the tannin-rich extract collection tank and adapted to forma concentrated extract from the tannin-rich liquid extract.